1. Field of the Invention
The present invention relates to a shift register circuit and a drive control apparatus, and more particularly to a shifter register circuit which can be excellently applied in a scanning driver of, e.g., an image display apparatus or an image read apparatus, and a drive control apparatus including this shift register circuit.
2. Description of the Related Art
In recent years, information devices such as a computer, a mobile phone or a personal digital assistance, or imaging devices such as a digital video camera, a digital still camera or a scanner have considerably gained in popularity. In such devices, an image display circuit for, e.g., a liquid crystal display panel or an image read circuit for, e.g., a photosensor array has been in heavy usage.
For example, an active matrix drive type liquid crystal display apparatus has a display panel in which display pixels (liquid crystal pixels) provided with pixel transistors each formed of a thin film transistor are arranged in a matrix form. The panel includes scanning lines through which the respective display pixels are connected in a row direction and data lines through which the same are connected in a column direction. In this apparatus, a scanning driver (a gate driver) is used to sequentially set the respective scanning lines to a selective state. A data driver is also used to apply a signal voltage corresponding to display data to the respective data lines. As a result, an alignment state of a liquid crystal in each display pixel set to the selective state is controlled, thereby display desired image information.
Here, the scanning driver is configured to generate and output a scanning signal which is used to sequentially set the respective scanning lines to the selective state, and is generally constituted of a shift register circuit.
There is also known an image read apparatus provided with a photosensor array in which photosensors (read pixels) are arranged in a matrix form the read apparatus also has a scanning driver which is used to sequentially set the photosensors in respective rows to a drive state (a selective state) at the time of an image read operation of the photosensor array. This apparatus is configured to read detection data (brightness data) corresponding to a light receiving quantity detected by each photosensor set to the drive state by a read driver, and acquire image information of an object. In such an image read apparatus, like the liquid crystal display apparatus mentioned above, the scanning driver is provided with a shift register circuit which generates and outputs a scanning signal which is used to sequentially set the photosensors in the respective rows to the drive state.
The scanning driver applied in the image display apparatus or the image read apparatus mentioned above will now be briefly described.
FIG. 13 is a structural view of a primary part showing an example of a scanning driver (a shift register circuit section) applied in a liquid crystal display apparatus according to a prior art.
The scanning driver applied in an image display apparatus (a liquid crystal display apparatus) has a configuration in which, e.g., a plurality of stages (shift blocks) SRC (q−1), SRC (q), SRC (q+1), . . . (q is an integer which is not smaller than 2) are cascade-connected as shown in this figure. The driver is provided with a shift register circuit section which sequentially inputs (transfer) an output signal from each stage SRC (q) to the next stage SRC (q+1). Here, the output signal from each stage SRC (q) is sequentially output as a scanning signal GOUT (k) to a scanning line in a corresponding row in accordance with the transfer operation (which converts a level of the output signal into a predetermined signal level), and inputs to a previous stage SCR (Q−1) as a reset signal.
In the scanning driver (the shift register circuit) shown in FIG. 1, reference characters CKV and CKVB denote clock signals having a mutual reversal relationship; STV, a shift start signal which is input to a non-illustrated first stage SRC (1); an END, a reset signal which is input to a last stage.
In the image display apparatus including such a scanning driver, according to a known display drive control method, since an operating frequency of the scanning driver can be generally set to be lower than that of a data driver, even a transistor element using a semiconductor material having relatively low electron mobility such as amorphous silicon or zinc oxide (ZnO) can be applied as a switching element constituting the scanning driver (the shift register circuit section).
In this case, when an element structure (a thin film transistor structure) using amorphous silicon or the like is applied to display pixels arranged in a display panel, these display pixels (the display panel) and a display drive device such as a scanning driver or a data driver which is a peripheral circuit can be integrally formed on a single panel substrate (a glass substrate or the like) by using the same manufacturing process. As a result, there has been studied/developed a technology which reduces a scale and a thickness of the apparatus and simplifies the manufacturing process to decrease a cost, for example.
As described above, in the thin film transistor element formed of a semiconductor material such as amorphous silicon or zinc oxide, its electron mobility is low and its operation characteristics are poor as compared with a thin film transistor element formed of a semiconductor material such as single-crystal silicon or polysilicon. However, in a case where this thin film transistor element is applied in the above-described image display apparatus or image read apparatus, it can be applied to a scanning driver which has an operating frequency lower than that of a data driver but has no problem in operation.
However, in the scanning driver to which an amorphous silicon transistor or the like is applied, since its operating frequency is essentially low, it is disadvantageously hard to be applied to a panel having a large number of scanning lines and a high operating frequency, e.g., a display panel having a high resolution or a large screen or a sensor array.
Specifically, it is generally known that an operating frequency (i.e., an operating speed) of the scanning driver is determined by a product (a time constant) of a resistance component (an output resistance) and a load carrying capacity of an output section of a scanning signal (i.e., an output section of each shift block (stage) constituting a shift register circuit). Here, the load carrying capacity is a sum of wiring capacities which are parasitic on the respective scanning lines, an input capacity in the next shift block and others, and the resistance component is an on resistance or the like of a switching element constituting the output section of the shift block.
As described above, when the thin film transistor such as an amorphous silicon transistor is applied in the scanning driver, a capacity component which serves as the load carrying capacity is large because of element characteristics, and an on resistance is low. Therefore, considerably reducing the operating frequency to be lower than that of the scanning driver to which a single-crystal silicon transistor is applied is unavoidable.
Further, the amorphous silicon transistor has characteristics that its manufacturing process is simple and uniform and excellent element characteristics can be obtained during manufacture, but deterioration in the element characteristics over time is larger than that in the single-crystal silicon transistor or the polysilicon transistor. Therefore, it has a problem that excellently performing display driving or read driving for a long time is difficult. Specifically, according to a verification by the present inventors, there has been obtained an experimental result that an operating frequency is reduced to approximately ½ of that in the initial state when an acceleration experiment is conducted for several-hundred hours in a temperature environment of approximately 80° C. There is a problem that excellent display driving or read driving cannot be assured in an actual product for a long period of time.